# Frequencie converter

#21

@BreakingDabs Using valves as to close (making smaller) the orifece so the amount Capable of passing treu lowers ?

#22

I have worked with hundreds of AC motors and VFDs and such in my career as control systems engineer.

The torque of the motor can be obtained from a data sheet for the motor if you contact the manufacturer. The peak torque will fall into a band specified on that data sheet. Generally the motors on a VFD operate at a constant torque mode up until a point. Horsepower is simply torque multiplied by RPM and the motors operate in a constant horsepower mode after they reach a critical RPM. Since horsepower equals torque times RPM then mathematically torque must fall off as RPM increases. So once past a certain RPM the horsepower remains constant but torque falls off a cliff on the charts as the motor transitions into constant horsepower mode.

The problem to cope with on a VFD is owing to electrical slip of the rotating magnetic field at lower RPM. When the electric field slips it generates current in the motor at unwanted spots and this tranlates into heat. So trying to run these motors under load for extended periods at low RPM causes heat build up not normally seen on a standard motor. This means that unless the motor is specified as a VFD duty motor that early malfunction rrom excessive heat is an issue.

VFDs have utility when extremely precise motor control is needed. Most motors run at 3450 rpm which accounts for slip of a 3600 cycle you would normally have from the standard 60 hz motor. VFDs change the frequency so that the magnetic fields used rotate at a different RPM than 3450. VFDs are also useful for converting single phase input power into a three phase motor if needed although at a reduced horsepower rating. This is possible because when a technique known as pulse width modulation (PWM) is used in a VFD as the means to control voltage then that voltage must first be converted into one central power bus. All AC power from each input phase is converted into DC and stored on the bus. The VFD then slices off just enough power to create each phase of motor power and using pulse width modulation then creates the sine wave in AC that each motor phase needs to see. For single phase input power simply wire the single phase input power with a jumper so the VFD βseesβ power at each input which it then converts to bus voltage.

For a vacuum pump application I am uncertain what advantage is gained by using a VFD that cannot be achieved without valving. VFDs are notoriously vulnerable to bad input power and also if more than one VFD is used in a system without installing both line and load reactors (3%) then you will be plagued with nuisance faults because this will be sensed as a voltage error.

I have designed many systems around VFDs but also worked around the RPM problem using outside means which is always preferred. The odd thing anout VFD systems is how picky one VFD can be about incoming line power quality while another right beside it is not. VFDs can be used typically up to 120 hz as well but as mentioned the torque drops like a rock after the motor hits its design RPM. You are asking though about a low rpm situation so I focused on that.

#23

Thank you @Beaker!!

good thing we be running -60C ethanol though the pump head then

#24

Yep thats it